Muscular SystemMuscular System

PA 481 CPA 481 C

Anatomy & Physiology Anatomy & Physiology

Tony Serino, Ph.D.Tony Serino, Ph.D.

Muscular System

• Functions: Movement –generation of

force and/or shortening Maintenance of posture Joint stabilization Heat Generation

Attributes: contractility, irritability,

extensibility, and elasticity

Types of Muscle Cells

Skeletal Muscle –voluntary, striated

Cardiac Muscle –involuntary, striated

Smooth Muscle –involuntary, no striations

Muscles wrapped with CT, that is continuous with tendon and periosteum

The elasticity of the CT sheaths, tendon and the muscle cells =

the Series Elastic Component

Antagonistic Muscle Arrangement

This arrangement plus the series elastic component allows the muscle to return to its original length.

Skeletal Muscle Cells• Long, cylindrical, non-branching, multinucleated• 10-100 mcm wide and up to 35 cm long• Voluntary, no spontaneous depolarization normally• Contractile proteins (myosin & actin) arranged in

bundles called myofibrils

Develop as a fusion of myoblasts, which accounts for multinucleated cells, extra myoblasts remain as satellite cells.

Unique Muscle Cell Structures

Sarcomere

Each skeletal muscle cell must be innervated by a

motor neuron to begin contracting.

Neuronal AP triggers release of ACh at

neuromuscular junction (motor end plate).

Neuromuscular Junction(Motor End Plate)

ACh is released and diffuses across gap

ACh bind to the nicotinic receptor and triggers a MEPP

The MEPP triggers an AP that races along the sarcolemma and down the T-tubules.The depolarization affects the SR cisternae which releases Ca++ into the cytoplasm.

The rise of intracellular Ca++ triggers the mechanical events of contraction.

Muscle Cell Contraction (Excitation-Contraction Coupling)

• A motor neuron is stimulated to fire an AP• AP reaches synaptic terminal triggering an influx

of Ca++

• The Ca++ stimulates the release of ACh• ACh diffuses across cleft and binds to nicotinic

receptors in motor end plate• This causes Na+ channels to open; causing the

generation of a MEPP• The MEPP triggers an AP along sarcolemma

and into T-tubules• This deplorarizes the SR cisternae which

releases stored Ca++ into the cytoplasm

Each myofibril consists of overlapping thick and thin filaments arranged in units called sarcomeres.

Muscle Contraction: Mechanical Events (Sliding Filaments)

• Calcium ions from SR flood the myofibrils

• This causes the thick and thin filaments to bind to each other (generates tension) and may cause them to slide past each other

• This causes the sarcomere to shorten

H Band

M Line Z Line

Myofibril Anatomy

Myofibril Structure (cross section)

Cross sections:

H Band M Line

Overlapping Tick and Thin Filaments

Thick Filament Structure

Thin Filament Structure: Twisted bead chain of actin proteins

Thin Filament: Actin, Tropomyosin and Troponin

Calcium is trigger

Detachment

Contraction Events

Detachment

Reset: energize myosin head

Detachment

Reset

Attachment

Detachment

Reset

Attachment

Power Stroke

Muscle Contraction Review

Muscles are arranged as Motor UnitsMotor Unit = 1 motor neuron + all the muscle fibers it controls (innervates)

The size of the motor unit depends on the degreeof control needed in that particular whole muscle.

Biomechanics of Force Production

• Tension = force exerted on an object by a muscle

• Load = force exerted on muscle by the weight of an object

• Twitch = the mechanical response of a muscle to an AP

• Types of Contractions:• Isometric = muscle increases

tension without shortening• Isotonic = muscle shortens with no

further increase in tension Load

TensionBicep

Fulcrum(pivot point) Weight of arm + object

Single Muscle Twitch

Factors Affecting Muscle Fiber Performance

Load –affects velocity of contraction• Increasing load decreases velocity

Frequency of stimulation Initial Length of muscle fiber Type of muscle fiber –fibers differ in

strength, size, ATP splitting rate, and resistance to fatigue

Load Effect on Degree andDuration of Contraction

Load vs. Velocity of Contraction

Factors Affecting Muscle Fiber Performance

Load –affects velocity of contraction• Increasing load decreases velocity

Frequency of stimulation Initial Length of muscle fiber Type of muscle fiber –fibers differ in

strength, size, ATP splitting rate, and resistance to fatigue

Mechanical (Wave) Summation

Increase frequency of stimulation allows tension to add to previous contraction’s tension

Factors Affecting Muscle Fiber Performance

Load –affects velocity of contraction• Increasing load decreases velocity

Frequency of stimulation Initial Length of muscle fiber Type of muscle fiber –fibers differ in

strength, size, ATP splitting rate, and resistance to fatigue

Initial Length of Muscle Fiber: affects the maximum tension that can be developed due to degree of overlap between thick and thin filaments

Factors Affecting Muscle Fiber Performance

Load –affects velocity of contraction• Increasing load decreases velocity

Frequency of stimulation Initial Length of muscle fiber Type of muscle fiber –fibers differ in

strength, size, ATP splitting rate, and resistance to fatigue

Types of Muscle Fiber: each motor unit consists of only one type of muscle fiber

• Slow twitch, red (oxidative) fibers (SO) –small diameter, weakest, slow ATPase, much myoglobin and mitochondria, abundant blood supply, fatigue resistant

• Fast twitch, red (oxidative) fibers (FO) –medium diameter, moderate strength, fast ATPase, abundant mitochondria and myoglobin, good blood supply, moderate fatigue resistance

• Fast twitch, white (glycolytic) fibers (FG) –largest diameter, great strength, fast ATPase, low amount of myoglobin or mitochondria, decreased blood supply, high in glycolytic enzymes, tire quickly

Control of Whole Muscle Tensiondependent on:

• Tension developed by each fiber– Dependent on fiber type, initial length and

degree of wave summation

• Amount of fibers stimulated to contract– The number of motor units responding is

directly related to amount of tension produced– If the body needs more power, it recruits more

motor units to respond– Known as recruitment (motor unit summation)

Energy Use: stored ATP in muscle used quickly so re-supply is crucial to function

1. Creatine Phosphate –quick re-supply, allowing time for aerobic respiration to gear up

2. Aerobic Respiration –oxidative phosphorylation dependent on adequate blood supply of oxygen, uses different sources for energy:

a) Stored glycogen

b) Glucose and fatty acids from blood

c) Fatty acids from blood

3. Anaerobic Respiration-becomes dominant as need for oxygen exceeds ability of blood to transport it into muscles

After exercise, energy continues to be consumed at increased levels to re-build reserves, etc., this is the oxygen debt incurred during the exercise

Fatigue –inability to maintain contraction tension even while being stimulated. Two kinds:

• Primary Fatigue –due to accumulation of lactic acid in sarcoplasm, this changes the cytoplasm pH and begins to change protein configurations which ends contraction.

• Secondary Fatigue –related to the loss of energy reserves in the body, as seen in day after soreness. Why this triggers a low intensity pain signal (a dull ache) is unknown.

Cardiac MuscleStriated, single nucleus,branched cells, connectedtogether by intercalateddiscs (with many gap junctions)

Spontaneously contracts, needs no innervation,involuntary

Smooth MuscleNo sarcomeres, therefore, no striations, single nucleated, small spindle shaped cells

Spontaneously contracts,involuntary control, can remaincontracted for long periods oftime without fatiguing

Two types:Visceral (single unit)

–united by gap junctionsMulti-unit –needs innervations, behaves like

skeletal muscle (Ex. Iris)

Smooth Muscle Cell

Visceral Smooth Muscle